The present invention relates to a process for producing a bodied silicone resin and more particularly the invention relates to producing a bodied silicone resin without the use of a catalyst.
Silicone resins are well known in the art, and specifically those composed of monofunctional units and difunctional units and having a silanol content of anywhere from 0.5% weight or less, to as much as 12%; see for instance Merrill et al., U.S. Pat. No. 3,865,766. Such silicone resins are utilized for a variety of purposes such as electrical protective coatings, the formation of laminates, as a basic constituent in silicone molding compounds, as water repellant coatings, and as intermediates in paints. In certain applications, such as for use in paints and as coatings for electrical encapsulation it is highly desirable that the uncured resin, after it is prepared and before it is utilized, to form the coating or to produce a silicone paint that the silicone resin be bodied; that is, that some of the excess hydroxyl groups be removed from the silicone resin such that it will have a silanol content between 2 to 4% by weight and such that some of the low molecular weight material of the silicone resin interreacts to form high molecular weight silicone resin polymers. This is known as bodying the resin, and various procedures have been set forth for bodying silicone resins.
As stated previously, it is desirable that a silicone resin be bodied for certain applications such that the resin does not have a large amount of low molecular weight material in it if it is to be used in paints or electrical encapsulating coatings. The reason for this is that the bodied silicone resin will have the proper consistency as a coating when it is applied to encapsulate electrical components, so that it will not burn off or drip off the electrical components prior to final cure and so that the silicone paint, specifically silicone polyester resin paints, will have the proper consistency and will cure to its final cure when it is applied to form a coating without the silicone paint running off the surface to which it is applied. There are various procedures for bodying silicone resins for the applications of producing electrical encapsulating coatings and as intermediates for silicone paints. All these various procedures involve the use or addition of a catalyst to the silicone resin and heating the silicone resin at elevated temperatures above 100.degree. C so as to interact and further condense hydroxyl groups from the silicone resin so as to form high molecular weight polymer species in the silicone resin, thus resulting in the bodying of the resin without necessarily the curing of the silicone resin and specifically without the final curing of the silicone resin.
The use of such catalysts being added to the silicone resin for bodying purposes has the disadvantage that these catalysts cannot be removed from the silicone resin and may interfere in subsequent applications of the bodied silicone resin as an intermediate in forming silicone paints or in forming electrical encapsulating coatings in which the silicone resin has to be finally cured with what may be a different type of incompatible catalyst.
In addition, with respect to such bodied silicone resins, which are bodied with the use or addition of a catalyst, it has been found that such silicone resins bodied by prior art methods had the disadvantage that they were insoluble in aliphatic solvents. It is preferred to utilize silicone resins in aliphatic solvents for certain end uses. For instance, in certain geographical locations the use of certain aliphatic solvents is specified for pollution control. Prior art bodied silicone resins were slightly, if at all, soluble in aliphatic solvents. As a result in the use of these boded silicone resins, they had to be dissolved in aromatic solvents which resulted in some difficulties with pollution standards in some parts of the United States.
It is true that there are some alkoxylated silicone resins which show more solubility in aliphatic solvents than the bodied silicone resins of the prior art. However, these silicone resins had to be heavily alkoxylated, that is heavily substituted with alkoxy groups. Even in those cases these alkoxylated silicone resins are not as highly soluble as would be desired in aliphatic solvents. In addition, it should be noted that in many instances in addition to electrical encapsulating and as intermediates in the production of silicone paints it is desired that silicone resins be bodied so as to form high molecular weight polymer species in the silicone resin mixture from as much of the low molecular polymer species as is possible so that substantial quantities of the resin will not volatilize during the curing of the silicone resin at elevated temperatures as is necessitated in many cases. For instance, prior art bodied silicone resins lost as much as 40% of the silicone resin content in volatiles upon being heated at 250.degree. C for any substantial period of time.
It is one object of the present invention to provide a process for bodying a silicone resin without the use of a catalyst.
It is another object of the present invention to provide a process for bodying a silicone resin without the use of a catalyst such that the volatiles given off by the silicone resin upon being heated to 250.degree. C, are substantially less than 40% by weight of the total silicone resin.
It is yet an additional object of the present invention to provide for bodied silicone resins having a silanol content of 2 to 4% by weight which are soluable in aliphatic solvents respective of the presence or absence of alkoxylate groups in the silicone resin.
Other objects of the present invention are accomplished in accordance with the disclosure set forth herein below.